The invention relates to a method for improving the Bayer process for the production of alumina trihydrate and alumina from bauxite ore. The invention concerns the use of cross-linked polysaccharides, specifically cross-linked dextran or cross-linked dihydroxypropyl cellulose to improve the performance of the filtration processes within the Bayer process.
In the typical Bayer process for the production of alumina trihydrate, bauxite ore is pulverized, slurried with caustic solution, and then digested at elevated temperatures and pressures. The caustic solution dissolves oxides of aluminum, forming an aqueous sodium aluminate solution. The caustic-insoluble constituents of bauxite ore, along with insoluble materials precipitated within the digestion processes, are together commonly referred to as red mud. Within the Bayer process the red mud solids are then separated from the aqueous liquor phase containing the dissolved sodium aluminate. This separation typically involves concentrating the bulk of the solids through settling tanks with flocculation, followed by filtration of the clarified overflow liquor to remove any residual solids remaining in the clarified liquor.
The concentrated underflow of red mud solids that result from the settling process are typically subjected to a series of washing steps to extract the remaining liquor that is present within the concentrated slurry. After the washing steps, the washed, concentrated solids slurry may then be subjected to filtration to remove excess liquid, prior to disposal or re-use of the solids.
As indicated above, the aqueous sodium aluminate solution, which has had the bulk of the insoluble red mud solids removed in the settling process after digestion, is typically subjected to a filtration step to remove any remaining insoluble material. The liquor resulting from this filtration is essentially free of any insoluble red mud residue. It is then cooled and solid alumina trihydrate product is precipitated out of the solution. The precipitated trihydrate is then most often separated into size fractions; larger particles being collected for further processing as product, while smaller sized particles are collected and re-used in the precipitation step as seed crystals. Again, the separation of the solid trihydrate particles from the liquor can be achieved using a number of means including settling, flocculation and filtration. Both product sized particles and seed particles may be subjected to some form of filtration to achieve appropriate separation from the liquor, which is subsequently returned to the digestion step of the process and re-used.
As described at least in part, among other places, in U.S. Pat. Nos. 6,814,873, 6,033,579, 6,048,463, and US Published Patent Application 2008/0257827, the Bayer process is constantly evolving and the specific techniques employed in industry for the various steps of the process not only vary from plant to plant, but also are often held as trade secrets.
While plant designs and operations may differ substantially and additional filtration steps can and do occur in various areas within the process, there is typically a number of key filtration steps used to assist or enhance the separation of solids from the liquid phases in Bayer process plants. Such filtration steps include, but are not limited to:                (a) Red Mud Security filtration—Filtration of primary settler overflow slurry        (b) Alumina Trihydrate Product filtration—Filtration of slurry containing product size alumina trihydrate        (c) Alumina Trihydrate Seed filtration—Filtration of slurry containing seed size alumina trihydrate        (d) Mud Filtration—Filtration of washed red mud slurry        
These filtration steps can include, but are not limited to, processes utilizing gravity, pressure, vacuum or a combination of these, as the underlying means of achieving filtration of the input slurry. Filtration steps may also involve washing of the filtered solids.
The operation and efficiency of each of the relevant filtration steps is a key factor in the effective operation of a Bayer process plant as a whole. As a result, there is a clear need and utility for methods to enhance and improve the filtration processes. Individual filtration processes may be enhanced by an improvement in the rate (throughput) of slurry through the filter and/or an improvement in the filtration efficiency (which may be considered as the quantity of liquid remaining with the solid material after filtration, as measured by residual liquor or moisture in the filtered solid cake). Such enhancements of the filtration processes are desirable across the range of filtration steps associated with the Bayer process.
The art described in this section is not intended to constitute an admission that any patent, publication or other information referred to herein is “prior art” with respect to this invention, unless specifically designated as such. In addition, this section should not be construed to mean that a search has been made or that no other pertinent information as defined in 37 CFR §1.56(a) exists.